The invention relates to an optical scanning device for scanning a first type of record carrier having a first information layer and a first transparent layer of a first thickness and for scanning a second type of record carrier having a second information layer and a second transparent layer of a second thickness different from the first thickness. The device includes a radiation source for generating a radiation beam and a lens system designed for converging the radiation beam through the first transparent layer to a focus on the first information layer. The invention also relates to a method for optically scanning a record carrier of the first or second type. The scanning includes writing, reading and/or erasing information in the record carrier.
The transparent layer in optical record carriers has, in general, the function of protecting the information layer from environmental influences and providing mechanical support for the record carrier, i.e. it acts as a substrate for the information layer. The thickness of the transparent layer is a compromise between the desired stiffness of the record carrier and the numerical aperture of the radiation beam used for scanning the information layer. If for a new type of record carrier the numerical aperture is increased in order to increase the storage density of the information layer, it is often necessary to reduce the thickness of the transparent layer in order to reduce the influence of disc tilt on the quality of the radiation beam. As a consequence, there will be different types of record carrier on the market, having different thicknesses of the transparent layer. A compatible record player should be able to scan the different types of record carrier, irrespective of the thickness of the transparent layer.
The transparent layer, through which a radiation beam scans the information layer, introduces a so-called spherical aberration in the radiation beam. The spherical aberration may be compensated in the lens system, making the radiation beam near its focus substantially free from spherical aberration. If a lens system compensated for a first thickness of the transparent layer is used for scanning a record carrier with a transparent layer of a second, different thickness, the quality of the focus will be deteriorated due to the under- or over-compensated spherical aberration.
The PCT application IB96/00182 to which Braat et al. U.S. 5,708,638 corresponds, describes a device for scanning optical record carriers of the first and second type. This device uses a lens system designed for converging a radiation beam through the first transparent layer to a best focus on the first information layer. When scanning a record carrier of the second type, the lens system forms a paraxial focus on the second information layer. The best focus of a beam is the point along the axis of the beam which has the highest intensity. The paraxial focus of a beam is the point along the axis of the beam through or towards which the paraxial rays of the beam are converged. The radiation reflected from the record carrier is detected by a radiation-sensitive detection system. When scanning a record carrier of the first type, the detection system uses all radiation in the reflected beam or the radiation in an outer annular area of the cross section of the reflected beam. When scanning a record carrier of the second type, the detection system detects only radiation from a central area of the cross section of the radiation beam. Since the lens system is not designed for converging a radiation beam through the thickness of the second transparent layer, the radiation beam will incur uncorrected spherical aberration on passage through the second transparent layer. By restricting the detection to the rays in a central area of the beam, the highly aberrated rays in the outer annular area of the beam will then have a reduced influence on the output signals of the detection system.